1. Field of the Invention
This invention relates to an optical information record disk, and more particularly to an optical information record disk of the type wherein a spacing is formed adjacent an internal signal record layer.
2. Description of the Prior Art
In an optical information record disk such as, for example, a DRAW (Direct Read After Write) disk, protection of a layer in which pits are formed, that is, a signal record layer is important because information is recorded as a large number of fine pits in the layer of the disk by a laser beam spot. Thus, an optical disk of the so-called "air sandwiching type" has been developed which includes a pair of circular base plates, at least one of which has a signal record layer formed thereon, and which are joined together in an opposing relationship such that the signal record layer may be located inside the disk.
In such a DRAW disk, a pair of circular base plates made of PC or PMMA are joined together in an opposing relationship with an internal spacing left therebetween, and a pair of signal record layers are located on inner faces of the transparent base plates. The base plates are adhered to each other by a layer of a bonding agent with a spacing left between the base plates by means of a pair of annular spacers at inner and outer circumferences of the disk. In such a DRAW disk, the area over which the annular spacers and each of the base plates are adhered to each other is small and is restricted to a portion of the disk over the signal record layer. Such a small area of adhesion between each of the base plates and the annular spacers results in a problem that the adhered portions of the disk may readily be exfoliated when the disk is acted upon by an impact force.
Thus, a bonding agent having a suitable visco-elasticity is used for the adhesive layer so as to act as a buffer for preventing such exfoliation of the disk caused by an impact force.
However, where there is a molding strain on surfaces of a pair of annular spacers at which the annular spacers are to contact with a base plate, adhesive layers may be extremely reduced in thickness at some portions thereof. Thus, it is a disadvantage that a desired buffer effect cannot be attained at such very thin portions of the adhesive layers of the disk and accordingly the disk may suffer from exfoliation at the very thin portions of the adhesive layer.